The conductivity in the material, he says, is now a million times higher than that of the starting material, and a thousand times higher than anything previously reported using a metal-organic framework.
Sandia National Laboratories researchers have devised a novel way to realize electrical conductivity in metal-organic framework (MOF) materials, a development that could have profound implications for the future of electronics, sensors, energy conversion and energy storage.
A paper to appear in Science magazine, “Tunable Electrical Conductivity in Metal-Organic Framework Thin-Film Devices,” debuted in the Dec. 5 edition of Science Express. The paper — co-authored by a group of Sandia researchers and collaborators at the National Institute of Standards and Technology(NIST) — describes a technique that experiments show successfully increases the electrical conductivity of one MOF by over six orders of magnitude.
“Fundamentally, this sheds enormous light on the conduction process in these materials,” said Alec Talin, a material scientist at Sandia and the paper’s lead author.
Applications for electrically conducting MOFs, said Sandia senior scientist Mark Allendorf, include chemical sensing, medical diagnostics, energy harvesting and storage and microelectronics.
MOFs: “Tinkertoys” for chemists
Materials researchers have considered MOF materials primarily for use in gas storage, drug delivery and other conventional applications for porous materials. Their crystalline structure, which resembles molecular scaffolding, consists of rigid organic molecules linked together by metal ions. This hybrid of inorganic and organic components produces an unusual combination of properties: nanoporosity, ultrahigh surface areas and remarkable thermal stability, which are attractive to chemists seeking novel materials that combine the superior performance of traditional inorganic semiconductors with the low cost and ease of fabrication typical of conducting organic polymers.
Allendorf, a chemist and MOF expert who called the research findings the most exciting development in his 28-year Sandia career, likens them to “tinker toys” for chemists.
“When you imagine the ‘Tinkertoys’ we played with as children, you recall they are essentially wooden balls with holes that you can link together with sticks,” Allendorf explained. “MOFs work the same way, only you substitute metal ions for the balls and organic molecules for the sticks.”
The resulting open space within the scaffolding can be filled with guest molecules, which gave Sandia’s Talin the idea to use the pore to make the MOFs electrically conducting.
“Importantly, MOFs possess a characteristic of molecules that allows us to adapt their properties to a specific application: we can perform chemistry on them, unlike traditional inorganic electronic materials, such as silicon and copper,” said Talin. Molecules, he said, represent the “ultimate, small-scale unit” at which electronic devices can be made. They are so difficult to manipulate and organize, however, that practical “molecular electronics” have not been realized. “How you connect to molecules, where you place them — those issues have consistently perplexed materials scientists,” said Talin.